**Abstract**

The objective of this study was to determine glucose isomerase activity in different prepared original or synthetic wine media to prevent sluggish or stuck fermentation, which may be caused by sugar uptake deficiency in yeast. The unfermented grape juice contains almost equal amounts of glucose and fructose. After fermentation, the residual sugar is mostly fructose, this is called glucose/fructose discrepancy (GFD) and is caused by the affinity decrease of hexose transporters towards fructose as ethanol accumulates. This results in stuck fermentation and is unwanted as the wine is sweet and risks microbial spoilage. Converting remaining fructose to glucose by glucose isomerase may be a solution so we tested the activity of this enzyme in synthetic and original wine media. Glucose formation, 0.5 % w/v, from 1% w/v fructose took place in synthetic wine medium containing 13 % v/v ethanol, 1% w/v glycerol and at pH 3.3. In original wine medium glucose formation did not take place except when wine was diluted at least five folds and at pH values equal or higher than 6 whether if tartaric acid was present or not. Since neither dilution, nor pH adjustment can be applicable, other ways to employ this enzyme should be tried.

**Keywords:** glucose/fructose discrepancy, stuck fermentation, glucose isomerase, fermentation, yeast, wine media
